A denitration catalyst to be used for an SCR method is generally prepared by adsorbing and supporting a vanadium oxide on anatase titania and adding an oxide of tungsten, molybdenum or the like thereto. The catalyst has a peak of an active reaction temperature in a range of about 350° to 400° C., and its denitration performance is lowered with a rise in reaction temperatures at 400° C. or higher. A typical characteristic of the catalyst is shown in FIG. 9. The catalyst exhibits a rapid lowering of denitration performance particularly at 450° C. or higher, and formation of NOx due to combustion of ammonia is observed at 500° C. or higher.
Recently, there have been some exhaust gas systems which need denitration by the SCR method at temperatures of 450° to 600° C. such as a gas turbine and the like equipped with no waste heat recovery boiler. A method which can denitrate an exhaust gas effectively at temperatures of 450° to 600° C. without cooling the exhaust gas and catalysts to be used for this method are required.
For denitration in a high-temperature region, a selective reduction method of nitrogen oxides without a catalyst (NCSR method) is known, but an optimum temperature in this method is 700° to 800° C., and its denitration performance is practically at most 60%.
A catalyst wherein a tungsten oxide is supported on titania and a catalyst wherein the tungsten oxide is supported on a zeolite carrier have so far been proposed for the purpose of use in the above-mentioned temperature region.
However, since denitration performance of these catalysts is apparently lower than that of the vanadium oxide-supporting denitration catalyst to be used in an ordinary temperature range, they require a large amount thereof, injection of an excessive reducing agent, injection of particular additives into the exhaust gas or the like in order to obtain a denitration rate of 90% or higher.
A subject of the present invention is to provide a method of enabling denitration of the exhaust gas effectively at temperatures of 450° to 600° C. and catalysts to be used for this method.